During the processing of a photographic element containing an imagewise exposed silver halide emulsion layer, reduced silver can be formed as a function of exposure. At the same time, at least a low level formation of reduced silver also occurs independently of imagewise exposure.
The term "fog" is herein employed to indicate the undesired formation of developed silver--or of developed dye in the case of color photography--in non-exposed areas.
Such fog is related to the fact, known to the man skilled in the art, that sensitivity centers of the silver halide grains may be rendered developable in the absence of light exposure. Fog generally increases with emulsion sensitivity, thus impairing the quality of the obtained photographic material.
In color photography the problem may be particularly serious, since a colored fog is generally more visible than a black and white fog.
The problem of reduction of fog formation may be reduced with substances which have the property of decreasing only fog without significantly reducing sensitivity. Such substances are reactivity associated with the photographic emulsion by introducing them into the layer containing said emulsion.
To eliminate this problem, various materials have been introduced into the photographic silver halide emulsion. Antifog agents have been described since the beginning of photography, as one can read for instance in "Stabilization of Photographic Silver Halide Emulsion", by E. J. Birr (Focal Press).
The most common materials to prevent fog formation are listed in Research Disclosure, Vol. 176, December 1978, No. 17643. Brooker et al., in U.S. Pat. No. 2,131,038, disclose thiazolium salts, including a simple cyanine dye, as useful antifogging agents. Mifune et al., U.S. Pat. No. 4,237,214, disclose benzothiazolium salts having quaternary substituents that can contain a carbamoyl or a sulfamoyl group as antifoggants. It has been generally recognized in the art that a particulary useful class of antifogging agents is represented by alkyl (including allyl) substituted thiazolium, benzothiazolium, selenazolium and benzoselenazolium salts. On the other hand, quaternary oxazolium salts are not generally known to be useful as antifogging agents. Arai et al., U.S. Pat. No. 3,954,478, disclose a silver halide emulsion containing alkenylbenzothiazolium salts as latent image stabilizers.
Problems are sometimes encountered with the use of antifogging agents. These problems relate to reduction of sensitivity caused by antifoggants when used in amounts effective to inhibit fogging. Therefore there is the need to provide compounds that inhibit fog formation during manufacture and storage of silver halide photographic materials, which do not cause a marked reduction in sensitivity.